Adiponitrile (ADN) is a commercially important and versatile intermediate in the industrial production of nylon polyamides useful in forming films, fibers, and molded articles. ADN may be produced by hydrocyanation of 1,3-butadiene (BD) in the presence of transition metal complexes comprising various phosphorus-containing ligands. For example, catalysts comprising zero-valent nickel and monodentate phosphorus-containing ligands are well documented in the prior art; see, for example, U.S. Pat. Nos. 3,496,215; 3,631,191; 3,655,723 and 3,766,237; and Tolman, C. A., McKinney, R. J., Seidel, W. C., Druliner, J. D., and Stevens, W. R., Advances in Catalysis, 1985, Vol. 33, pages 1-46. Improvements in the hydrocyanation of ethylenically unsaturated compounds with catalysts comprising zero-valent nickel and certain multidentate phosphite ligands are also disclosed; e.g., see: U.S. Pat. Nos. 5,512,696; 5,821,378; 5,959,135; 5,981,772; 6,020,516; 6,127,567; and 6,812,352.
3-Pentenenitrile (3PN) may be formed through a series of reactions as illustrated below.

According to abbreviations used herein, BD is 1,3-butadiene, HC≡N is hydrogen cyanide, and 2M3BN is 2-methyl-3-butenenitrile. A method to increase the chemical yield of 3PN from BD hydrocyanation includes the catalytic isomerization of 2M3BN to 3PN (Equation 2 above) in the presence of NiL4 complexes as disclosed in U.S. Pat. No. 3,536,748. Co-products of BD hydrocyanation and 2M3BN isomerization may include 4-pentenenitrile (4PN), 2-pentenenitrile (2PN), 2-methyl-2-butenenitrile (2M2BN), and 2-methylglutaronitrile (MGN).
In the presence of transition metal complexes comprising various phosphorus-containing ligands, dinitriles such as ADN, MGN, and ethylsuccinonitrile (ESN) may be formed by the hydrocyanation of 3PN and 2M3BN, as illustrated in Equations 3 and 4 below. Equation 4 also shows that 2M2BN can be formed when 2M3BN undesirably isomerizes in the presence of a Lewis acid promoter that may be carried over from a pentenenitrile hydrocyanation reaction zone.

The hydrocyanation of activated olefins such as conjugated olefins (e.g., 1,3-butadiene) can proceed at useful rates without the use of a Lewis acid promoter. However, the hydrocyanation of un-activated olefins, such as 3PN, require at least one Lewis acid promoter to obtain industrially useful rates and yields for the production of linear nitriles, such as ADN. For example, U.S. Pat. Nos. 3,496,217, 4,874,884, and 5,688,986 disclose the use of Lewis acid promoters for the hydrocyanation of non-conjugated ethylenically unsaturated compounds with nickel catalysts comprising phosphorous-containing ligands.
An integrated process for the production of ADN from BD and HC≡N can comprise BD hydrocyanation, 2M3BN isomerization to produce 3PN, and the hydrocyanation of pentenenitriles, including 3PN, to produce ADN and other dinitriles. Integrated processes are disclosed, for example, in United States Patent Application 2009/0099386 A1.
Disclosed in United States Patent Publication No. 2007/0260086, is a process for the preparation of dinitriles with an aim to provide for the recovery of a catalyst formed by a mixture of mono- and bidentate ligands and to be able to reuse the catalyst thus recovered in the hydrocyanation and/or isomerization stages.
United States Patent Publication No. 2008/0221351 discloses an integrated process for preparing ADN. A first process step includes hydrocyanating BD to produce 3PN over at least one zero-valent nickel catalyst. A second process step of the integrated process involves hydrocyanating 3PN to produce ADN over at least one zero-valent nickel catalyst and at least one Lewis acid. In this integrated process, at least one of the zero-valent nickel catalysts used in one of the process steps is transferred into the other process step.
In order to optimize the use of catalyst, recycle of catalyst is desirable. However, undesirable substances, such as catalyst degradation products and reaction byproducts, may end up in catalyst recycle loops along with catalyst. When such catalyst degradation products and reaction byproducts are recycled along with catalysts, they tend to build up and cause loss of product yield and purity. Accordingly, there is a need to recycle catalysts, while retaining optimal product yield and purity. There is also a need to replenish catalyst which has been lost by degradation.